Process for producing tempered steel motor vehicle parts

ABSTRACT

A process for production of tempered steel support structures for motor vehicles of multiple steel parts including the steps deforming and/or trimming at least two soft-hardened steel parts, hardening the at least two steel parts, joining the steel parts to form a support structure, wherein the support structure is subject to annealing and total stress relieving in a combined thermal treatment only after the joining step.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention concerns a process for producing support structures for motor vehicles from multiple steel parts of air hardened and tempered steel including the steps deforming and/or trimming at least two soft-annealed steel parts, hardening the at least two steel parts, and joining the steel parts to form a support structure, and in particular, a process for the production of high-strength zinced vehicle components of air hardened steel.

2. Description of Related Art

Chassis and load bearing or support structures for motor vehicles are subject to very different local load conditions. They include for example front and rear axle carriers, door jams or posts, bumpers, and longitudinal and transverse complex geometric components of locally different rigidity and stiffness requirements. A one piece component can not be design engineered to adequately meet these different requirements. Instead, the wall thickness of the entire component is frequently determined using the highest rigidity requirements, and the less loaded areas are thus accordingly over-dimensioned.

In order to circumvent these problems, use is increasingly made of so-called tailored blanks, in which sheet metal of varying thicknesses are joined to each other prior to shaping. By the arrangement of the sheets and their thickness relationship, the rigidity characteristics in the deformed component can be locally targeted to match the requirements. This manufacturing of the tailored blanks is however expensive and the processing in the case of complex components is in part very awkward.

In DE 101 20 934 A1, for the manufacture of chassis and undercarriage components with load adapted component construction, it is proposed to construct the parts of multiple components with differing stiffnesses adapted to the respective local load situation. The components located in highly loaded area are therein heat tempered. Preferably, heat tempered and non tempered components are assembled to make the final component. In practice it has been found that it is frequently necessary to anneal these components for total stress reduction. This is in particular necessary when the component is to be hot galvanized, since otherwise tears or cracks could be formed. This means, a supplemental thermal step.

SUMMARY OF THE INVENTION

It is the task of the invention to provide an economic manufacturing process for motor vehicle components of hardened steel wherein least two parts are joined.

The task is inventively solved by a process for production of tempered steel support structures for motor vehicles from multiple steel parts wherein an annealing and total stress relieving of the steel parts in the shape of the support structure is carried out in a combined thermal treatment only after joining.

DETAILED DESCRIPTION OF THE INVENTION

In accordance with the invention a process with the following sequence of process steps is provided:

-   -   deforming and/or trimming at least two annealed steel parts,     -   hardening the at least two steel parts,     -   joining the steel parts into a support structure, and     -   after joining, carrying out an annealing and total stress         relieving of the steel parts in the shape of the support         stricture in a combination thermal treatment.

Therein it is of substantial significance for the invention that, in contrast to the conventional processes, it is not the individual components that are hardened and tempered, but rather the hardening and tempering occurs first after the joining of the components to the finished component has been concluded. Thus, the individual component parts are essentially hardened, and only later, after joining, are they tempered by annealing and total stress relieving. The tempering and hardening is thus first carried out with the joined component.

This matter of proceeding has the particular advantage, that the process of the annealing of the individual components can be omitted. The process chain is thus substantially shortened thereby.

The hardening is a thermal treatment that is comprised of forming an austenitic phase and rapid quenching—with the goal of forming a martensitic phase.

Depending upon the composition of the steel, the hardening process can be varied. Transformation hardening is only possible with materials which, during cooling, exhibit the y/a-transformation. This is not possible for pure austenitic or ferritic steels. A precondition for an increase in hardening is the presence of at least 0.2% carbon, which is dissolved in the gamma-iron, but with rapid transformation cannot migrate or diffuse out and is suspended in the ferrite lattice. Since the mobility of the carbon is greater in unalloyed steels than in many alloyed (Mn—, Cr—, Ni—) steels, unalloyed steel must be quenched with a high rate of temperature drop, e.g. in water. In contrast, alloyed steel is cooled in oil or, as the case may be, air.

The age hardening takes advantage of the temperature-dependent solubility characteristic of the iron lattice for certain impurities or foreign atoms. Saturated mixed crystals are quenched and, at room temperature, express the excess foreign atoms which can no longer be dissolved. These place the lattice under tension and thus lead to an increase in rigidity.

The tensions resulting from hardening are reduced by the annealing.

The tempering is a thermal treatment consisting of hardening and annealing, preferably above 550° C. Rigidity is increased by the transformation hardening, at the same time however the microstructure is newly formed and refined. Although the previously achieved increase in rigidity is partially again lost during annealing, on the other hand the toughness is increased beyond the original value. In accordance with the invention the annealing occurs only after the joining of the hardened parts, so that the tempering is carried out only after the joining.

A further advantage of the invention is the combining of annealing and total stress relieving. During total stress relieving inner tensions, which occur during cooling of the steel piece, are released.

In accordance with the invention, only after joining is an annealing and a total stress reduction of the steel parts carried out in a combined thermal treatment. Preferably the temperature of the combined thermal treatment is from 550-690° C. Particularly preferred are temperatures of from 570-650° C. The duration of the thermal treatment is preferably from 30 minutes to two hours. Typically the hardening is carried out as air hardening.

After hardening and prior to annealing the individual parts are joined into a steel support structure. For this, the conventional joining processes can be employed. Preferably a welding process is selected. The inventive maimer of proceeding has therein the advantage, that the weld seam is also annealed in the combined thermal treatment of annealing and tempering. Thereby the quality of the weld seam is substantially improved.

In a preferred further development of the invention, following the combined thermal treatment, a galvanizing of the tempered steel support structure takes place. By the tempering and the total stress relieving hardly any interfering tears or cracks in the component are formed.

A further variant envisions that following the combined thermal treatment the support structure is subject to a cathodic dip coating (CDC). The preferred steel support structures include chassis elements and undercarriage elements of the motor vehicles. A preferred variant at least one sheet shaped and one tubular shaped steel part are employed as the components. The steel parts can be of different thickness. As the tubular shaped steel part, in particular a component deformed by internal high pressure (IHU) can be employed.

It is advantageous when, prior to the combined thermal treatment of the support structure, joining parts and/or joining means for securing to other components are affixed. The joining parts could in particular be screws, rivets or nuts and the joining means and can be comprised in particular of bore holes, screw-in sleeve sockets, or crimps or grooves.

Particularly preferred is when the joining means are already affixed prior to hardening. Thereby the processing for incorporation of the joining means is substantially simplified. It is particularly preferred when the process is employed to produce high strength and zinced chassis or frame parts of air hardened steel. Alloyed steel is in particular employed for the steel parts. 

1. A process for production of tempered steel support structures for motor vehicles of multiple steel parts including the steps deforming and/or trimming at least two soft-annealed steel parts, hardening the at least two steel parts, joining the steel parts to form a support structure, wherein an annealing and total stress relieving of the steel parts in the shape of the support structure is carried out in a combined thermal treatment only after joining.
 2. A process according to claim 1, wherein the combined thermal treatment is carried out in a temperature range of from 550 to 690° C.
 3. A process according to claim 1, wherein the combined thermal treatment is carried out in a temperature range of 570 to 650° C.
 4. A process according to claim 1, wherein the hardening is air hardening.
 5. A process according to claim 1, wherein the joining is by welding.
 6. A process according to claim 1, wherein the support structure is galvanized following the combined thermal treatment.
 7. A process according to claim 1, wherein the support structure is cathartically dip coated (KTL) following the combined thermal treatment.
 8. A process according to claim 1, wherein at least one sheet-shaped and one tubular-shaped steel part are employed.
 9. A process according to claim 1, wherein at least two steel parts of different thickness are employed.
 10. A process according to claim 1, wherein joining parts and/or joining means for securing of other components to the support structure are affixed to the support structure prior to the combined thermal treatment.
 11. A process according to claim 8, wherein the joining parts are screws, rivets or nuts and the joining means are bore holes, screw-in sleeve sockets or folds or creases.
 12. A process according to claim 1, wherein the steel parts are alloyed steel.
 13. A process for production of tempered steel support structures for motor vehicles of multiple steel parts including the steps deforming and/or trimming at least two soft-annealed steel parts, air hardening the at least two steel parts, joining the air hardened steel parts to form a support structure, annealing and total stress relieving the support structure in a combined thermal treatment after joining, and galvanizing the support structure subsequent to the combined thermal treatment. 